Human infections caused by Staphylococcus aureus present a serious therapeutic challenge due to the emergence of antibiotic-resistant strains. Of major concern are infections with methicillin-resistant S. aureus (MRSA), highly virulent microorganisms and the most common infectious disease in American hospitals. MRSA have acquired resistance mechanisms to all known antibiotics and many isolates are broadly resistant against most antiinfective agents. Future research must aim at understanding the molecular biology of MRSA pathogenesis and the development of specific vaccines that prevent MRSA infectious diseases. This proposal reveals a specialized secretion system of S. aureus that is involved in the pathogenesis of human and animal infections. The S. aureus ess (ESAT-6 secretion system) locus consists of a cluster of eight genes, three of which (esxA, esxB and esaC) encode products that are secreted by a mechanism requiring the machinery genes essABC. During infection, both murine and human hosts generate humoral immune responses to EsxA, EsxB and EsaC, suggesting that all clinical S. aureus isolates engage Ess secretion in vivo. We show that S. aureus causes persistent infections in experimental animals, similar to staphylococcal disease in humans. The Ess pathway is required for the pathogenesis of staphylococcal infections, as esxB mutants are unable to form abscesses or persist in host tissues, whereas mutants that impact EsaC secretion initially replicate but then fail to persist. Animals first infected with wild-type S. aureus remain susceptible to subsequent staphylococcal infection, whereas animals infected with esxB mutants develop immunity to subsequent S. aureus infections. Here we will explore the Ess pathway of S. aureus to unravel the molecular basis of abscess formation and persistent infections. Moreover, staphylococcal esxB mutants are interrogated for the genetic requirements of generating protective immunity against S. aureus disease. PUBLIC HEALTH RELEVANCE: Staphylococcus aureus is the leading cause of bloodstream, lower respiratory tract, skin and soft tissue infections in the United States with annual morbidity of about 3 million and annual mortality approaching 100,000 Americans lives. Our research into the development of chronic-persistent infections and the contribution of the ESAT-6 secretion system in preventing the development of protective immunity against S. aureus are designed to reveal the molecular basis of pathogenesis as well as the targets of protective immunity.